1. Field of the Invention
The invention relates to a lamp for generating colored light, preferably red or amber light, the lamp having an interference filter coating and a light absorbing coating formed on the exterior of its transparent envelope. Such a lamp is used for taillights, brake lights or indicator lights, daytime running lights or parking lights of motor vehicles.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
European laid-open specification EP 0 986 093 A1 discloses an incandescent lamp whose transparent lamp vessel has an interference filter coating and a light absorbing coating for emitting red or amber light. The light absorbing coating consists of a single layer of iron oxide Fe2O3 having a thickness in the range of 5.4 nanometer to 5.8 nanometer.
European laid-open specification EP 1 156 514 A1 (U.S. Pat. No. 6,661,164) describes an incandescent lamp whose transparent envelope has an interference coating and a light absorbing coating for emitting red light. The light absorbing coating comprises two layers of iron oxide Fe2O3 separated by one layer of silicon dioxide wherein the thickness of the iron oxide layers is 8 nanometer and 14 nanometer respectively.
U.S. Pat. No. 5,200,855 describes the use of multi-layer interference coatings where the layers of high optical refraction of the interference coating are formed from iron oxide Fe2O3 which is a material with high absorption in the shorter visible wavelengths. The referenced patent does not teach the utility of using deliberately non-uniform thickness of the absorbing material to provide the desired color purity while minimizing lumen reductions due to the absorbing layer. Lamp envelopes coated with an interference filter according to the afore-said patent would have undesirably low lumen output due to the absorption of the multiple layers of absorbing material.